1. Field of the Invention
This invention relates generally to methods and apparatus for performing ultrasonic diagnosis of a target body. More particularly, the invention pertains to methods and apparatus for measuring compressibility or compliance in a target body. The invention is directed towards techniques for enhancing the accuracy of such measurements in compressible or compliant targets, particularly the human body, using one or more ultrasonic transducers in pulse-echo mode.
2. Description of Related Art
Traditional ultrasonic diagnosis is achieved by transmitting ultrasonic energy into a target body and generating an image from the resulting echo signals. A transducer is used to both transmit the ultrasonic energy and to receive the echo signals. During transmission, the transducer converts electrical energy into mechanical vibrations. Acquired echo signals produce mechanical oscillations in the transducer which are reconverted into electrical signals for amplification and recognition.
A plot or display (e.g., on an oscilloscope, etc.) of the electrical signal amplitude versus echo arrival time yields an amplitude line (A-line) or echo sequence corresponding to a particular ultrasonic transmission. When the A-line is displayed directly as a modulated sinusoidal pattern at radio frequency ("RF"), it is typically referred to as an RF or "undetected" signal. For imaging, the A-line is often demodulated to a non-RF or "detected" signal.
Ultrasound techniques have been extensively used in the field of diagnostic medicine as a non-invasive means of analyzing the properties of tissue in vivo (i.e., living). A human or animal body represents a nonhomogeneous medium for the propagation of ultrasound energy. Acoustic impedance changes at boundaries of regions having varying densities and/or sound speeds within such a target body. At such boundaries, a portion of the incident ultrasonic beam is reflected. Inhomogeneities within the tissue form lower level scatter sites that result in additional echo signals. Images may be generated from this information by modulating the intensities of pixles on a video display in proportion to the intensity of echo sequence segments from corresponding points within the target body.
Conventional imaging techniques are widely used to evaluate various diseases within organic tissue. Imaging provides information concerning the size, shape, and position of soft tissue structures using the assumption that sound velocity within the target is constant. Qualitative tissue characterization is carried out by interpretation of the grey scale appearance of the sonograms. Qualitative diagnosis largely depends on the skill and experience of the examiner as well as characteristics of the tissue. Images based only on relative tissue reflectivity, however, have limited use for quantitative assessment of disease states.
Techniques for quantitative tissue characterization using ultrasound are needed for more accurate diagnosis of disorders. In recent years many significant developments have been achieved in the field of ultrasonic tissue characterization. Some acoustic parameters, e.g., speed of sound and attenuation, have been successfully used for tissue characterization. One promising physical parameter for quantitative measurement is compressibility or compliance. The amount of compressibility or compliance within tissues changes within regions of varying density. Diseased tissue, such as tumors, may be harder or softer than normal tissue, and thus have a different amount of compressibility.
Tissue compressibility is an important parameter which is used to detect the presence of diffuse or localized disease. Measuring changes in compressibility becomes important in the analysis of tissue for pathological conditions. Many tumors are firmer than the surrounding normal tissue, and many diffuse diseases result in firmer or more tender pathology. Examples can be found in diffuse liver disease, prostate cancer, uterine fibroids, muscle conditioning or disease, and many other conditions.
Traditionally, physicians routinely palpate various regions of a patient's body to get an impression of tissue firmness or tissue softness. This technique is a form of remotely trying to sense what is going on in terms of tissue compliance. For example, in a liver, if the compliance in an area is sensed to be different from compliance in the surrounding area, the physician concludes from the tactile sensations in his fingers that something is wrong with the patient. The physician's fingers are used to perform a qualitative measurement.
The ability to quantitatively measure the compressibility or compliance of tissue in localized regions would help with (1) objective quantification of commonly used clinical signs, (2) localizing these measures, (3) making the measurements deep in tissue with simple equipment, (4) constructing images of the compressibility or compliance parameter in vivo, which may be used alone or in conjunction with ordinary sonograms.
One technique has attempted to quantitatively measure the elasticity and compressibility of tissues by correlating patterns obtained in ultrasonic measurements of tissue movement in vivo. The method applies Fourier analysis to a clinical study of patterns of tissue movement, specifically in the liver. The technique uses Fourier analysis to enable objective differentiation of different tissue types in pathologies on the basis of numerical features of the time-course of the correlation coefficient between pairs of A-scans recorded with a particular time separation. Tissue oscillations resulting from periodic stimulus by waves resulting from ventricular contraction and pressure pulses in the descending aorta are measured to derive patterns of movement. Fourier series transformation is used to analyze the data to quantitate the kinetic behavior of the tissue in vivo. See. Tristam et al.. "Application of Fourier Analysis to Clinical Study of Patterns of Tissue Movement," Ultrasound in Med. & Biol., Vol. 14, No. 8, (1988) 695-707.
In another approach, patterns of tissue movement are correlated in vivo. This technique basically studies details of the patterns of movement in tissues in response to a normal physiological dynamic stimulus such as cardiac motion. A method is given for quantifying tissue movement in vivo from the computation of a correlation coefficient between pairs of A-scans with appropriate time separation. Tristam et al., "Ultrasonic Study of in vivo Kinetic Characteristics of Human Tissues," Ultrasound in Med. & Biol., Vol. 12, No. 12 (1986) 927-937.
The waveforms of liver dynamics caused by aortic pulsation and vessel diameter variations are analyzed in still another method, involving a signal processing technique for analyzing radio frequency M-mode signals. The technique uses patterns of movement in response to arterial pulsation to determine tissue characteristics. The technique measures displacement, velocity, and strain as a function of time in small deformations in tissue due to arterial pulsation. Wilson and Robinson, "Ultrasonic Measurement of Small Displacements and Deformations of Tissue," Ultrasonic Imaging, Volume 4, (1982) 71-82.
Yet another method processes echoes in order to measure tissue motion in vivo. The motion patterns observed in vivo are correlated to arterial pressure pulse. Dickinson and Hill, "Measurement of Soft Tissue Motion Using Correlation Between A-Scans," Ultrasound in Med. & Biol., Vol. 8, No. 3, (1982) 263-271.
All of the above techniques focus upon the dynamic motions of tissue in vivo. These methods are limited due to the complexity of tissue motion, and the behavior of the stimuli employed in those methods.